Various products commercially available are sold in large capacity, board or kraft paper packaging typically characterized as an "economy size" packaging. Illustrative of these types of products are soaps, detergents, etc.
Although smaller packagings have portions thereof which are perforated so as to be able to have the perforated portion pushed inward in order to create a discharge aperture, larger capacity packagings commonly are opened by drawing a tear tab along at least a portion of the periphery of the packaging box in order to, for example, provide a hinged lid providing access to the contents carried therein.
Such larger capacity packagings are typically made from folding carton board stock or heavy kraft paper. The board stock or paper is run over rollers or between nips formed between pairs of rollers in an "in-machine" direction. As the board stock or kraft paper passes a particular position, apparatus at the position feeds a tab string or tape material from a spool onto the moving substrate. The tab material is impregnated with a thermoplastic adhesive which, prior to the tab being applied to the moving substrate, is heated to a temperature at which it will become soft enough so that, when it is brought into engagement with the substrate, it will wet-out sufficiently to allow bonding of the tab to the substrate.
Various devices in the prior art have utilized heated rolls or wheels to effectuate the heating, and consequent softening, of the adhesive impregnating the tab. U.S. Pat. No. 3,617,422 issued to John Rene Paulson on Nov. 2, 1971 illustrates one particular hot wheel applicator.
Structures of this type, however, have a number of inherent problems. A significant limitation upon the productivity of a facility manufacturing packaging as previously described is the speed at which the web substrate moves. The speed of the substrate is, however, limited by the speed at which the tab can be fed into engagement therewith and, in turn, the speed at which the thermoplastic adhesive can be brought to a desired temperature at which the tab can be satisfactorily bonded to the substrate. The governing manufacturing limitation, therefore, becomes the speed at which the adhesive is brought to its application temperature.
In conducting manufacturing operations, the web substrate can be made to travel at speeds between 500 feet per minute and 1000 feet per minute. If a heating wheel such as that illustrated in the Paulson patent were used to heat the adhesive prior to applying the tab to a substrate moving this fast, the adhesive could not be brought to a sufficiently high temperature in order to form a satisfactory bond between the tab and the substrate.
The case is similar with other wheel and roll type heating applicators. One type of applicator commercially available utilizes a heating roll having a diameter of 16 inches. When such a roll is utilized to apply a tab to a substrate moving at 500 feet per minute, a particular segment of the tab is in engagement with, and absorbing heat from, the roll for a maximum of approximately 0.4 seconds. This time period is inadequate to wet-out most adhesives available for this application in order that a satisfactory bond be formed.
In reviewing the Paulson patent, one notices that, with that structure, the adhesive which is being heated is in engagement with the heating roll about only 75 to 80% of the circumference of the roll. The period of time during which the adhesive is heated, or dwell time, as it is referred to, can be increased by causing the adhesive to engage the heating roll so that a larger percentage of the circumferential surface of the roll will provide heat to the adhesive. Because of the engagement of the heated roll by the moving substrate, however, 80% of the circumferential surface is the approximate practical limit which can be made to be engaged by the adhesive.
Another possible solution to the problem is increasing the size of the heating roll in order to increase dwell time. This solution, however, also proves to be impractical. The 16 inch diameter wheel currently used in some structures is approaching the maximum size which can practicably be used, and, because of space limitations, use of a larger wheel would frequently be precluded.
Since the heat which is transmitted from the heated wheel to the impregnated tab is proportional to the temperature differential which exists between the wheel and the adhesive, one other possible solution is suggested. This solution would increase the temperature of the wheel to a level above that to which the adhesive is desired to be heated. Consequently, the adhesive would be provided with sufficient heat in order to insure adequate bonding even if the dwell time were as little as 0.4 seconds.
This solution, however, presents its own problems. Non-uniform bonding would be effected should the speed of the substrate in the "in-machine" direction be varied. Even more significantly, if movement of the substrate is terminated, the tab might become overheated. Where a tab having a thermoplastic core such as polyester is used, not only might the adhesive be melted, but the core also might be melted to the point where the tab breaks.
A further reason why it is not desirable to heat the wheel above the temperature to which the adhesive should be heated is the consequent reduction in service life of the equipment because of the higher operating temperatures. With such elevated temperatures, the equipment is subject to a shorter life. For these reasons, therefore, elevating the heating element to a temperature no higher than that to which the adhesive is desired to be raised is considered desirable.
It is to these problems and shortcomings in the prior art that the invention of the present application is directed. It provides a structure by which a tab, to be applied to a moving web substrate, can be heated sufficiently in order to insure adequate bonding to the substrate, but without overheating the tab to a point at which it will break. Additionally, it allows the tab to be heated uniformly along its length regardless of the speed at which the substrate is made to move in an "in-machine" direction.